The paper focuses on the nomadic pastoralist Kel Adrar Twareg of north Mali, who divide their year into three approximately equal seasons: a hot rainy season, a cold dry season, and a hot dry season. The rainfall, which is highly variable, is the product of the West African monsoons, the pastoral environments being at the extremity of the monsoon range. Plant production is more or less directly related to total annual rainfall. The effects of climatic seasonality are modified by animal breeding cycles. Camels, which have a gestation period of one year, are given a calving interval of 24 months, which increases the period of lactation, and enhances the chance of the calf's survival. The breeding of other animals is also manipulated. Among sheep this involves ensuring that young are born when there is adequate pasture available, so generally only one lambing season in the year. Goats have different requirements and are sometimes born after the end of the rainy season when the tree and shrub growth is still sufficient, and this allows for two kid crops a year. The arrangement of breeding periods allows for lactation to be spread reasonably evenly through the year, apart from during the hot dry season. Camels and goats are superior to sheep and cattle for subsistence purposes, as their lactation period is longer. This should be borne in mind by development agencies. Millet is bought or bartered using animals or salt carried from the Tawdenni mines. Grain storage problems mean that it is not necessarily possible for them to buy and sell when the prices are in their favor; generally they have to buy millet in the hot dry season when the price is highest. Pastoralism is more vulnerable to seasonality than sedentary agriculture, and, in order to mitigate this, pastoralists adjust their demographic regulation, their meat consumption and their practice of alternative activities. Rich pastoralists are better able to cope with drought than the poor, in that they can raise different livestock species, and have more contacts for assistance. During times of shortage, they are also in a position to loan a lactating animal or to make other gifts; this serves a redistributive purpose in minor seasonal crises, but when the situation worsens they have a lot of favors to call in. Seasonal variations have always been an aspect of pastoral life, and the crisis in the Sahel cannot be attributed to them. The problem that is facing pastoralists is that the external economy, on which they are increasingly dependent, and which acts as a drain on the resources and services of the pastoral system. Seasonality may compound this in that economic differentiation is hastened, and an increasing number of pastoralists are put in the position, during severe dry spells, to sell their animals and access rights and seek wage employment. Accordingly, development policy should target fundamental variables such as control of land and water, in that it is through such management that productivity can be increased and benefits distributed over the year. Sedentarization will accentuate the effects of seasonality unless it is accompanied by irrigation and/or fodder storage. The provision of banking or credit schemes or grain storage facilities would allow pastoralists to buy grain when the price is low.